Duchenne muscular dystrophy is an X-linked, progressive degenerative disease of skeletal muscle of unknown etiology. Many studies have suggested the presence of a fundamental abnormality in cell membranes. Using fibroblasts as a model system, we have identified a set of abnormalities in cells from patients with DMD. These changes include: (i) decreased activity of dipeptidylaminopeptidase-I (DAP-I), a chloride-requiring lysosomal protease; (ii) decreased expression of latent DAP-I activity following disruption of the lysosomal membrane; (iii) an apparent increase in the chloride content and/or chloride permeability (PC1) of the lysosome. These observations have been unified in a hypothesis suggesting that the etiology of DMD involves an increased chloride permeability of the cell, including both the lysosomal and the plasma membranes. The hypothesis has been tested experimentally and support has come from experiments showing that the PC1 of DMD fibroblasts is significantly increased (p less than 0.0001) and that increased permeability of the muscle membrane to Cl- is capable of causing a loss of muscle membrane excitability. The research proposed here is designed to further investigate this hypothesis by identifying and characterizing the specific Cl- channels involved. A computer generated model of the muscle action potential will be used to investigate possible compensating responses of muscle to a Cl--induced loss of excitability. In the course of studies on DAP-I, we have also found that its activity is reduced in fibroblasts from female subjects. These data strongly suggest that DMD is a Y-influenced X-linked disease rather than a simple X-linked disorder. This genetic model predicts that androgen and/or estrogen hormones should affect DAP-I activity and chloride permeability and that they should be affected differently in DMD and control cells. This prediction will be examined by studying the DAP-I activity and Cl- permeability of DMD fibroblasts in response to administration of androgen and estrogen hormones. If correct, this modification to the genetics of DMD has immediate applicability to possible modes of therapeutic intervention. Our long term goals are to understand the etiology of DMD at the molecular level and to use knowledge to elucidate the normal regulatory processes of muscle development and function.